Supporting cooperative transmission in massive multiple-input multiple-output (mimo) systems

ABSTRACT

Embodiments of the disclosure relate to supporting cooperative transmission in massive multiple-input multiple-output (MIMO) systems, such as a wireless distribution system (WDS). A WDS includes a plurality of remote units each defining a home coverage cell. A selected remote unit can coordinate with a neighboring remote unit(s) to help mitigate inter-cell interference for a selected client device(s) located in an overlapping coverage area between the home coverage cell of the selected remote unit and a neighboring coverage cell(s) defined by the neighboring remote unit(s). The selected remote unit receives channel-data information from the selected client device(s) and forms a first radio frequency (RF) beam based on the channel-data information to distribute a downlink signal(s) to the selected client device. The selected remote unit coordinates with the neighboring remote unit(s) based on the channel-data information to form a second RF beam to distribute the downlink signal(s) to the selected client device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/692,771, filed Aug. 31, 2017, which claims the benefit of priorityunder 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No.62/464,751 filed on Feb. 28, 2017, the content of which is relied uponand incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates generally to supporting cooperative transmissionin a massive multiple-input multiple-output (MIMO) system, such as awireless distribution system (WDS).

Wireless customers are increasingly demanding digital data services,such as streaming video signals. At the same time, some wirelesscustomers use their wireless communications devices in areas that arepoorly serviced by conventional cellular networks, such as insidecertain buildings or areas where there is little cellular coverage. Oneresponse to the intersection of these two concerns has been the use ofDASs. DASs include remote units configured to receive and transmitcommunications signals to client devices within an antenna range of theremote units. DASs can be particularly useful when deployed insidebuildings or other indoor environments where the wireless communicationsdevices may not otherwise be able to effectively receive RF signals froma source.

In this regard, FIG. 1 illustrates a distribution of communicationsservices to coverage cells 100(1)-100(N) of a WDS provided in the formof a DAS 102, wherein ‘N’ is the number of coverage cells. Thesecommunications services can include cellular services, wirelessservices, such RF identification (RFID) tracking, Wireless Fidelity(Wi-Fi), local area network (LAN), and wireless LAN (WLAN), wirelesssolutions (Bluetooth, Wi-Fi Global Positioning System (GPS),signal-based, and others) for location-based services, and combinationsthereof, as examples. The coverage cells 100(1)-100(N) may be remotelylocated. In this regard, the coverage cells 100(1)-100(N) are created byand centered on remote units 104(1)-104(N) connected to a central unit106 (e.g., a head-end equipment, a head-end controller, or a head-endunit). The central unit 106 may be communicatively coupled to a signalsource 108, for example, a base transceiver station (BTS) or a basebandunit (BBU). In this regard, the central unit 106 receives downlinkcommunications signals 110D from the signal source 108 to be distributedto the remote units 104(1)-104(N). The remote units 104(1)-104(N) areconfigured to receive the downlink communications signals 110D from thecentral unit 106 over a communications medium 112 to be distributed tothe respective coverage cells 100(1)-100(N) of the remote units104(1)-104(N). Each of the remote units 104(1)-104(N) may include an RFtransmitter/receiver and a respective antenna 114(1)-114(N) operablyconnected to the RF transmitter/receiver to wirelessly distribute thecommunications services to client devices 116 within the respectivecoverage cells 100(1)-100(N). The remote units 104(1)-104(N) are alsoconfigured to receive uplink communications signals 110U from the clientdevices 116 in the respective coverage cells 100(1)-100(N) to bedistributed to the signal source 108. The size of each of the coveragecells 100(1)-100(N) is determined by the amount of RF power transmittedby the respective remote units 104(1)-104(N), receiver sensitivity,antenna gain, and RF environment, as well as by RF transmitter/receiversensitivity of the client devices 116. The client devices 116 usuallyhave a fixed maximum RF receiver sensitivity, so that theabove-mentioned properties of the remote units 104(1)-104(N) mainlydetermine the size of the respective coverage cells 100(1)-100(N).

The DAS 102 and the coverage cells 100(1)-100(N) can be configured tofunction as a massive MIMO system, in which the coverage cells100(1)-100(N) each form a respective micro coverage cell. The remoteunits 104(1)-104(N) in the coverage cells 100(1)-100(N) may be providedas low-power remote radio heads (RRHs) to distribute the downlinkcommunications signals 110D and/or receive the uplink communicationssignals 110U using the same RF spectrum (e.g., RF band/channel). In thisregard, it may be possible to adapt the DAS 102 to support the nextgeneration (e.g., fifth-generation (5G)) wireless communicationssystems.

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinency of any cited documents.

SUMMARY

Embodiments of the disclosure relate to supporting cooperativetransmission in massive multiple-input multiple-output (MIMO) systems,such as a wireless distribution system (WDS). A WDS includes a pluralityof remote units, which may provide similar wireless communicationsservices functionality to remote radio heads (RRHs) in the massive MIMOsystem. The remote units in the WDS each define a respective homecoverage cell among a plurality of coverage cells in the WDS. Inexamples disclosed herein, a selected remote unit in the WDS isconfigured to coordinate with a neighboring remote unit(s) to mitigateinter-cell interference for a selected client device(s) located in anoverlapping coverage area between the home coverage cell of the selectedremote unit and a neighboring coverage cell(s) defined by theneighboring remote unit(s). Specifically, the selected remote unitreceives channel-data information from the selected client device(s) andforms a first radio frequency (RF) beam based on the channel-datainformation to distribute a downlink signal(s) to the selected clientdevice. In addition, the selected remote unit coordinates with theneighboring remote unit(s) based on the channel-data information to forma second RF beam to distribute the downlink signal(s) from theneighboring remote unit to the selected client device. By distributingthe downlink signal(s) to the selected client device in the first RFbeam and the second RF beam (e.g., simultaneously), it is possible toimprove signal to interference and noise ratio (SINR) of the downlinksignal(s) received by the selected client device, thus mitigatinginter-cell interference for the selected client device. Further, byforming the first RF beam and the second RF beam based on thechannel-data information received from the selected client device, it isalso possible to reduce processing overhead and complexity associatedwith coordinated scheduling (CS) and/or coordinated beamforming (CB),thus helping to improve robustness and performance of the WDS.

In this regard, in one aspect, a WDS is provided. The WDS includes aplurality of remote units each configured to define a home coverage cellamong a plurality of coverage cells in the WDS to communicate one ormore downlink signals to one or more client devices located within aboundary of the home coverage cell. A selected remote unit among theplurality of remote units is configured to identify at least oneselected client device located in at least one overlapping coverage areabetween the home coverage cell of the selected remote unit and at leastone neighboring coverage cell defined by at least one neighboring remoteunit among the plurality of remote units. The selected remote unit isalso configured to receive channel-data information from the at leastone selected client device. The selected remote unit is also configuredto form a first RF beam based on the channel-data information todistribute at least one first downlink signal among the one or moredownlink signals to the at least one selected client device. Theselected remote unit is also configured to coordinate with the at leastone neighboring remote unit based on the channel-data information toform at least one second RF beam to distribute the at least one firstdownlink signal from the at least one neighboring remote unit to the atleast one selected client device.

In another aspect, a method for supporting cooperative transmissionsamong a plurality of remote units in a WDS is provided. The methodincludes identifying at least one selected client device located in atleast one overlapping coverage area between a home coverage cell of aselected remote unit among the plurality of remote units and at leastone neighboring coverage cell defined by at least one neighboring remoteunit among the plurality of remote units. The method also includesreceiving channel-data information from the at least one selected clientdevice. The method also includes forming a first RF beam based on thechannel-data information to distribute at least one first downlinksignal to the at least one selected client device. The method alsoincludes coordinating with the at least one neighboring remote unitbased on the channel-data information to form at least one second RFbeam to distribute the at least one first downlink signal from the atleast one neighboring remote unit to the at least one selected clientdevice.

Additional features and advantages will be set forth in the detaileddescription which follows and, in part, will be readily apparent tothose skilled in the art from the description or recognized bypracticing the embodiments as described in the written description andclaims hereof, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary and are intendedto provide an overview or framework to understand the nature andcharacter of the claims.

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate one or moreembodiments, and together with the description serve to explainprinciples and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary wireless distributionsystem (WDS), which may be a distributed antenna system (DAS) forexample;

FIG. 2A is a schematic diagram of an exemplary multi-user multiple-inputmultiple-output (MU-MIMO) coverage cell in which a remote radio head(RRH) is configured to support MU-MIMO communications with a pluralityof client devices;

FIG. 2B is a schematic diagram of an exemplary conventional network MIMOsystem in which a plurality of RRHs is configured to support MU-MIMOcommunications with a plurality of client devices in a plurality ofcoverage cells;

FIG. 3 is a schematic diagram of an exemplary WDS in which a pluralityof remote units is configured to support cooperative transmission in aplurality of coverage cells with reduced computational overhead andenhanced inter-cell interference mitigation compared to the conventionalnetwork MIMO system of FIG. 2B;

FIG. 4 is a flowchart of an exemplary process that can be employed by aselected remote unit among the remote units in FIG. 3 to supportcooperative transmission in the WDS;

FIG. 5A is a graph providing an exemplary illustration of userthroughput comparison between the conventional network MIMO system ofFIG. 2B and the WDS of FIG. 3 in which the remote units cooperate basedon matched-filter (MF) pre-coding;

FIG. 5B is a graph providing an exemplary illustration of userthroughput comparison between the conventional network MIMO system ofFIG. 2B and the WDS of FIG. 3 in which the remote units cooperate basedon zero-forcing (ZF) pre-coding;

FIG. 6 is a schematic diagram of an exemplary WDS provided in the formof an optical fiber-based WDS that can include the remote units of FIG.3 for supporting cooperative transmission in the WDS of FIG. 3;

FIG. 7 is a partial schematic cut-away diagram of an exemplary buildinginfrastructure in which a WDS, such as the WDS of FIG. 6, including theremote units of FIG. 3 for supporting cooperative transmission in theWDS of FIG. 3; and

FIG. 8 is a schematic diagram representation of additional detailillustrating an exemplary computer system that could be employed in acontroller, such as a control circuit employed by any of the remoteunits of FIG. 3, for supporting cooperative transmission in the WDS ofFIG. 3.

DETAILED DESCRIPTION

Embodiments of the disclosure relate to supporting cooperativetransmission in massive multiple-input multiple-output (MIMO) systems,such as a wireless distribution system (WDS). A WDS includes a pluralityof remote units, which may provide similar wireless communicationsservices functionality to remote radio heads (RRHs) in the massive MIMOsystem. The remote units in the WDS each define a respective homecoverage cell among a plurality of coverage cells in the WDS. Inexamples disclosed herein, a selected remote unit in the WDS isconfigured to coordinate with a neighboring remote unit(s) to mitigateinter-cell interference for a selected client device(s) located in anoverlapping coverage area between the home coverage cell of the selectedremote unit and a neighboring coverage cell(s) defined by theneighboring remote unit(s). Specifically, the selected remote unitreceives channel-data information from the selected client device(s) andforms a first radio frequency (RF) beam based on the channel-datainformation to distribute a downlink signal(s) to the selected clientdevice. In addition, the selected remote unit coordinates with theneighboring remote unit(s) based on the channel-data information to forma second RF beam to distribute the downlink signal(s) from theneighboring remote unit to the selected client device. By distributingthe downlink signal(s) to the selected client device in the first RFbeam and the second RF beam (e.g., simultaneously), it is possible toimprove signal to interference and noise ratio (SINR) of the downlinksignal(s) received by the selected client device, thus mitigatinginter-cell interference for the selected client device. Further, byforming the first RF beam and the second RF beam based on thechannel-data information received from the selected client device, it isalso possible to reduce processing overhead and complexity associatedwith coordinated scheduling (CS) and/or coordinated beamforming (CB),thus helping to improve robustness and performance of the WDS.

Before discussing exemplary aspects of cooperative transmission methodsin massive MIMO systems, a brief overview of a multi-user (MU) MIMO(MU-MIMO) coverage cell and a conventional network MIMO system includingmultiple MU-MIMO coverage cells is first provided with reference toFIGS. 2A and 2B, respectively. The discussion of specific exemplaryaspects of cooperative transmission methods in massive MIMO systemsstarts below with reference to FIG. 3.

In this regard, FIG. 2A is a schematic diagram of an exemplary MU-MIMOcoverage cell 200 in which an RRH 202 is configured to support MU-MIMOcommunications with a plurality of client devices 204(1)-204(4). In anon-limiting example, the RRH 202 is a low power radio transceiverlocated remotely from a central radio transceiver (e.g., base station,evolved Node-B, etc.) for communicating downlink and uplink RF signalsin a respective coverage cell such as the MU-MIMO coverage cell 200. TheMU-MIMO coverage cell 200 is defined by a cell boundary 205, which maybe a perimeter at a determined distance (e.g., radius) from the RRH 202.Although the MU-MIMO coverage cell 200 is shown to include only theclient devices 204(1)-204(4), it should be appreciated that the MU-MIMOcoverage cell 200 can include more than the client devices 204(1)-204(4)as shown in FIG. 2A.

The RRH 202 includes a plurality of antennas 206(1)-206(M) provided inan antenna array 208. As such, the RRH 202 is able to form a pluralityof RF beams 210(1)-210(4) for communicating a plurality of wirelesscommunications signals 212(1)-212(4) with the client devices204(1)-204(4), respectively.

To achieve higher spectral efficiency, the RRH 202 is configured to(e.g., simultaneously) communicate the wireless communications signals212(1)-212(4) to the client devices 204(1)-204(4) based on the sametime-frequency resource, which includes RF carriers in a frequencydomain and symbol streams in a time domain. A broadband wireless system,such as long-term evolution (LTE) for example, uses frequency and timeto spread data in the wireless communications signals 212(1)-212(4),providing high speeds and greater signal reliability. For eachsubcarrier, the data in the wireless communications signals212(1)-212(4) is sent in continuous symbols, each representing a definedduration in time. As such, intra-cell interference, such as co-channelinterference (CCI), may occur between the RF beams 210(1)-210(4), thuscausing severe degradation in the wireless communications signals212(1)-212(4). In this regard, it may be desired that the RRH 202perform control and/or coordination measures to mitigate the intra-cellinterference between the RF beams 210(1)-210(4). For example, the RRH202 can be configured to pre-code respective symbol streams for each ofthe wireless communications signals 212(1)-212(4) and/or control therespective RF power in each of the RF beams 210(1)-210(4) to helpsuppress the CCI and improve the SINR for each of the wirelesscommunications signals 212(1)-212(4).

A number of the MU-MIMO coverage cells, such as the MU-MIMO coveragecell 200, can be included in a large-scale system to form a network MIMOsystem for providing wireless communications services in an indoorand/or outdoor environment. In this regard, FIG. 2B is a schematicdiagram of an exemplary conventional network MIMO system 214 in which aplurality of RRHs 216(1)-216(4) is configured to support MU-MIMOcommunications with a plurality of client devices 218(1)-218(17) in aplurality of coverage cells 220(1)-220(4). The coverage cells220(1)-220(4) are defined by a plurality of cell boundaries221(1)-221(4), respectively. In this network MIMO system, each of theRRHs 216(1)-216(4) is functionally equivalent to the RRH 202 in FIG. 2A.

Although the conventional network MIMO system 214 is shown to includeonly the RRHs 216(1)-216(4) and the coverage cells 220(1)-220(4), itshould be appreciated that the conventional network MIMO system 214 caninclude more than the RRHs 216(1)-216(4) and more than the coveragecells 220(1)-220(4). In addition, it should be appreciated that theconventional network MIMO system 214 can also include more than theclient devices 218(1)-218(17) as shown in FIG. 2B. Notably, a practicalnetwork MIMO system may include dozens of RRHs and hundreds of clientdevices. As such, the RRHs 216(1)-216(4), the coverage cells220(1)-220(4), and the client devices 218(1)-218(17) are discussedherein as non-limiting examples.

Each of the RRHs 216(1)-216(4) is configured to function as the RRH 202of FIG. 2A to support MU-MIMO communications in a home coverage cellamong the coverage cells 220(1)-220(4). In examples discussedhereinafter, a coverage cell is referred to as a home coverage cell toan RRH if the coverage cell is identified by a correspondingidentification of the RRH. In contrast, a coverage cell not identifiedby the corresponding identification of the RRH, but which overlaps atleast partially with the home coverage cell identified by the RRH, canbe referred to as a neighboring coverage cell. In this regard, thecoverage cell 220(1) is the home coverage cell served by the RRH 216(1),the coverage cell 220(2) is the home coverage cell served by the RRH216(2), the coverage cell 220(3) is the home coverage cell served by theRRH 216(3), and the coverage cell 220(4) is the home coverage cellserved by the RRH 216(4).

In a non-limiting example, the coverage cells 220(1)-220(4) areneighboring coverage cells that overlap, at least partially, in aplurality of overlapping coverage areas 222(1)-222(4). As shown in FIG.2B, the coverage cell 220(1) overlaps with the coverage cell 220(2) inthe overlapping coverage area 222(1), the coverage cell 220(2) overlapswith the coverage cell 220(3) in the overlapping coverage area 222(2),the coverage cell 220(3) overlaps with the coverage cell 220(4) in theoverlapping coverage area 222(3), and the coverage cell 220(4) overlapswith the coverage cell 220(1) in the overlapping coverage area 222(4).In this regard, the coverage cells 220(2), 220(4) are neighboringcoverage cells to the coverage cell 220(1), the coverage cells 220(1),220(3) are neighboring coverage cells to the coverage cell 220(2), thecoverage cells 220(2), 220(4) are neighboring coverage cells to thecoverage cell 220(3), and the coverage cells 220(1), 220(3) areneighboring coverage cells to the coverage cell 220(4).

For the convenience of reference and illustration, the client devices218(1)-218(17) are divided into two categories, which are referred to as“in-cell client devices” and “cell-boundary client devices.” In examplesdiscussed hereinafter, “in-cell client devices” refer to client deviceslocated inside any of the coverage cells 220(1)-220(4), but outside anyof the overlapping coverage areas 222(1)-222(4). In this regard, asshown in FIG. 2B, the client devices 218(1), 218(6) are in-cell clientdevices of the coverage cell 220(1), the client devices 218(7)-218(9)are in-cell client devices of the coverage cell 220(2), the clientdevices 218(11)-218(13) are in-cell client devices of the coverage cell220(3), and the client devices 218(16), 218(17) are in-cell clientdevices of the coverage cell 220(4). In contrast, “cell-boundary clientdevices” refer to client devices located inside any of the overlappingcoverage areas 222(1)-222(4). In this regard, as shown in FIG. 2B, theclient devices 218(2), 218(3) are cell-boundary client devices insidethe overlapping coverage area 222(1), the client devices 218(10) is acell-boundary client device inside the overlapping coverage area 222(2),the client devices 218(14), 218(15) are cell-boundary client devicesinside the overlapping coverage area 222(3), and the client devices218(4), 218(5) are cell-boundary client devices inside the overlappingcoverage area 222(4).

With continuing reference to FIG. 2B, the RRHs 216(1)-216(4) include aplurality of antenna arrays 224(1)-224(4), respectively. Notably, eachof the antenna arrays 224(1)-224(4) can include a plurality of antennas225(1)-225(M) that can be configured to support MIMO and/or beamformingoperations as previously described in FIG. 2A in a respective homecoverage cell among the coverage cells 220(1)-220(4). Together, theantenna arrays 224(1)-224(4) and the multiple antennas 225(1)-225(M) ineach of the antenna arrays 224(1)-224(4) can form a distributed antennaarray to perform MU-MIMO operations (e.g., simultaneously) to the clientdevices 218(1)-218(17) located in the conventional network MIMO system214.

According to pervious discussions in FIG. 2A, each of the RRHs216(1)-216(4) is configured to communicate a plurality of wirelesscommunications signals 226(1)-226(K) in a plurality of RF beams228(1)-228(K). To maximize spectral efficiency throughout theconventional network MIMO system 214, each of the RRHs 216(1)-216(4) isconfigured to communicate the wireless communications signals226(1)-226(K) to the client devices 218(1)-218(17) based on the sametime-frequency resource, which includes RF carriers in a frequencydomain and symbol streams in a time domain. As such, as discussedearlier in FIG. 2A, intra-cell interference (e.g., CCI) may occurbetween the RF beams 210(1)-210(4), thus causing severe degradation inthe wireless communications signals 212(1)-212(4). Moreover, given thatthe conventional network MIMO system 214 can potentially include a largenumber of RRHs for supporting MU-MIMO operations in a large number ofcoverage cells, some of the client devices 218(1)-218(17), particularlythe cell-boundary client devices located in the overlapping coverageareas 222(1)-222(4), may also suffer inter-cell interferences from anRRH(s) in a neighboring coverage cell(s).

The wireless communications signals 226(1)-226(K) communicated by an RRH216(i) (1≤i≤N), wherein N represents the total number of RRHs (e.g., N=4as shown in FIG. 2B) in the conventional network MIMO system 214, can becollectively referred to as a transmitted signal s₁ and expressed in theequation (Eq. 1) below.

$\begin{matrix}{s_{i} = {{W_{i}a_{i}} = {{\sum_{k = 1}^{K}{w_{i,k}a_{i,k}}} = {\begin{bmatrix}w_{i,1} & w_{i,2} & \cdots & w_{i,K}\end{bmatrix}{\quad{\begin{bmatrix}a_{i,1} \\a_{i\; 2} \\\vdots \\a_{i,K}\end{bmatrix} = {\begin{bmatrix}w_{i,1,1} & w_{i,2,1} & \cdots & w_{i,K,1} \\w_{i,1,2} & w_{i,2,2} & \cdots & w_{i,K,2} \\\vdots & \vdots & \ddots & \vdots \\w_{i,1,M} & w_{i,2,M} & \cdots & w_{i,K,M}\end{bmatrix}\begin{bmatrix}a_{i,1} \\a_{i\; 2} \\\vdots \\a_{i,K}\end{bmatrix}}}}}}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

In the equation (Eq. 1) above, K represents the number of the clientdevices 218(1)-218(K), and therefore the number of the RF beams228(1)-228(K), in the coverage cell 220(i) of the RRH 216(i). Mrepresents number of the antennas 225(1)-225(M) in the RRH 216(i).Herein, each of the client devices 218(1)-218(K) in the coverage cell220(i) is assumed to include a single antenna. W_(i) represents an M×Kpre-coding matrix for the RRH 216(i), a_(i) represents a K×1 transmittedsymbol vector for the RRH 216(i), w_(i,k) represents an M×1 pre-codingvector for the client device 218(k) (1≤k≤K), and a_(i,k) representstransmitted symbol for the client device 218(k) by the RRH 216(i).

The transmitted signal s_(i) is received by the client device 218(k) inthe coverage cell 220(i) as a received signal x_(i,k), which can begeneralized in the equation (Eq. 2) below.

x _(i,k)=Σ_(j)√{square root over (ρ_(f))}H _(j,k) W _(j) a _(j) +n_(i)  (Eq. 2)

In the equation (Eq. 2) above, ρ_(i) represents the signal-to-noiseratio (SNR) of the received signal x_(i,k), n_(i) represents whiteGaussian noise with a power of one (1), and H_(j,k) represents a 1×Mchannel matrix between the client device 218(k) in the coverage cell220(i) and the antennas 225(1)-225(M) in the RRH 216(j) of the coveragecell 220(j) (1≤j≤N, j≠i). By substituting equation (Eq. 2) into equation(Eq. 1), the received signal x_(i,k) can be further expressed in theequation (Eq. 3) below.

x _(i,k)=√{square root over (ρ_(f))}H _(i,k) w _(i,k) a _(i,k)+√{squareroot over (ρ_(f))}H _(i,k)Σ_(m≠k) ^(K) w _(i,m) a _(i,m)+√{square rootover (ρ_(f))}Σ_(j≠i) ⁴Σ_(m=1) ^(K) H _(j,k) w _(j,m) a _(j,m) +n_(i)  (Eq. 3)

In the equation (Eq. 3) above, the first term √{square root over(ρ_(f))}H_(i,k)w_(i,k)a_(i,k) represents the desired wirelesscommunications signal received by the client device 218(k) in thecoverage cell 220(i), the second term √{square root over(ρ_(f))}H_(i,k)Σ_(m≠k) ^(K)w_(i,m)a_(i,m) represents intra-cellinterference, and the third term √{square root over (ρ_(f))}Σ_(j≠i)⁴Σ_(m=1) ^(K)H_(j,k)w_(j,m)a_(j,k) represents inter-cell interference.In this regard, in the conventional network MIMO system 214 of FIG. 2B,each of the client devices 218(1)-218(17), particularly those clientdevices located in the overlapping coverage areas 222(1)-222(4), can besubject to intra-cell and/or inter-cell interferences.

With continuing reference to FIG. 2B, the intra-cell and/or inter-cellinterferences experienced by each of the client devices 218(1)-218(17)can be seen as superposition of signals that were intended for otherclient devices in the conventional network MIMO system 214. In thisregard, if the wireless communications signals 226(1)-226(K)communicated by each of the RRHs 216(1)-216(4) can be controlled in thefrequency domain and the time domain, it may be possible to suppress theintra-cell and/or the inter-cell interferences to help detect andreceive the desired wireless communications signal represented by thefirst term of the equation (Eq. 3).

Theoretically, when a matched filter (MF) is used in each of the RRHs216(1)-216(4), which means w_(i,k)=H_(i,k)* and the number of antennas225(1)-225(M) in each of the RRHs 216(1)-216(4) increases to infinity,the intra-cell and the inter-cell interferences may disappear completelyif the RF channel between each of the client devices 218(1)-218(17) andeach of the RRHs 216(1)-216(4) follows independent Rayleighdistribution. However, it would be impossible for any of the RRHs216(1)-216(4) to include an infinite number of the antennas225(1)-225(M). In this regard, it is necessary to mitigate theintra-cell and inter-cell interferences to make the conventional networkMIMO system 214 practically usefully.

According to previous discussions in FIG. 2A, each of the RRHs216(1)-216(4) can be configured to mitigate intra-cell interference in arespective home coverage cell by pre-coding the respective symbolstreams for each of the wireless communications signals 226(1)-226(K)and/or controlling the respective RF power in each of the RF beams228(1)-228(K). Furthermore, the RRHs 216(1)-216(4) can be configured tomitigate the inter-cell interferences in the overlapping coverage areas222(1)-222(4) through CS and/or CB. More specifically, each of the RRHs216(1)-216(4) receives channel-data information (e.g., CSI) fromrespective client devices located in a respective home coverage cell. Ina non-limiting example, the channel-data information received via theCSI includes explicit and implicit physical channel feedback, such aschannel quality indication (CQI), pre-coding matrix indicator (PMI), andrank indicator (RI). The RRHs 216(1)-216(4) share the channel-datainformation received from all of the client devices 218(1)-218(17) inthe conventional network MIMO system 214 via a backbone network. Assuch, each of the RRHs 216(1)-216(4) can use the channel-datainformation related to all of the client devices 218(1)-218(17) topre-code the wireless communications signals 226(1)-226(K) in therespective coverage cell among the coverage cells 220(1)-220(4). Inaddition, the RRHs 216(1)-216(4) can coordinate among each other toensure that each of the client devices 218(1)-218(17) in theconventional network MIMO system 214 communicates with only one of theRRHs 216(1)-216(4). For example, for the client devices 218(2), 218(3)that are located in the overlapping coverage area 222(1), the RRHs216(1), 216(2) can cooperatively determine that the client device 218(2)will only communicate with the RRH 216(1), and the client device 218(3)will only communicate with the RRH 216(2). Accordingly, the RRHs216(1)-216(4) can be referred to as cooperative RRHs, and theoverlapping coverage areas 222(1)-222(4) can be referred to ascooperative areas.

With each of the RRHs 216(1)-216(4) being configured to cooperate basedon CS/CB in the conventional network MIMO system 214, the transmittedsignal s_(i) communicated by the RRH 216(i) (1≤i≤N) can be expressed inthe equation (Eq. 4) below.

$\begin{matrix}{s_{i} = {{W_{i}a_{i}} = {{\sum_{k = 1}^{N*K}{w_{i,k}a_{i,k}}} = {\begin{bmatrix}w_{i,1} & w_{i,2} & \cdots & w_{i,{N*K}}\end{bmatrix}{\quad{\begin{bmatrix}a_{i,1} \\a_{i\; 2} \\\vdots \\a_{i,{N*K}}\end{bmatrix} = {\begin{bmatrix}w_{i,1,1} & w_{i,2,1} & \cdots & w_{i,{N*K},1} \\w_{i,1,2} & w_{i,2,2} & \cdots & w_{i,{N*K},2} \\\vdots & \vdots & \ddots & \vdots \\w_{i,1,M} & w_{i,2,M} & \cdots & w_{i,{N*K},M}\end{bmatrix}\begin{bmatrix}a_{i,1} \\a_{i\; 2} \\\vdots \\a_{i,{N*K}}\end{bmatrix}}}}}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

In the equation (Eq. 4) above, W_(i) represents M×N*K pre-coding matrixfor RRH 216(i), in which M represents the number of the antennas225(1)-225(M) in the RRH 216(i), K represents number of the clientdevices 218(1)-218(K) in the coverage cell 220(i), and N represents thetotal number of RRHs (e.g., N=4 as shown in FIG. 2B) in the conventionalnetwork MIMO system 214. In addition, a_(i) represents an N*K×1transmitted symbol vector for the RRH 216(i), w_(i,k) represents an M×1pre-coding vector for the client device 218(k) (1≤k≤K), and a_(i,k)represents transmitted symbol for the client device 218(k) by the RRH216(i). In this regard, in the conventional network MIMO system 214 asshown in FIG. 2B, each of the RRHs 216(1)-216(4) needs to receive andprocess 4*K channel-data information. Accordingly, for a larger-scalenetwork MIMO system including N RRHs, each of the RRHs would need toreceive and process N*K channel-data information. As such, processingoverhead and complexity for the RRHs 216(1)-216(4) can becomeprohibitively high.

In the conventional network MIMO system 214, a received signal x_(i,k)received by the client device 218(k) in the coverage cell 220(i) can begeneralized in the equation (Eq. 5) below.

x _(i,k)=Σ_(j)√{square root over (ρ_(f))}H _(j,k) w _(j,k) a_(j,k)+Σ_(j)√{square root over (ρ_(f))}H _(j,k)Σ_(m≠k) ^(N*K) w _(j,m) a_(j,m) +n _(i)  (Eq. 5)

In the equation (Eq. 5) above, H_(j,k) represents a 1×M channel matrixbetween the client device 218(k) in the coverage cell 220(i) and theantennas 225(1)-225(M) in the RRH 216(j) of the coverage cell 220(j)(1≤j≤N, j≠i). When the large-scale factor of H₁,k between the RRHs216(1)-216(4) is similar, it may be possible to maximize a combinationgain of the desired wireless communications signal, which is representedby the first term Σ_(j)√{square root over (ρ_(f))}H_(j,k)w_(j,k)a_(j,k)in the equation (Eq. 5), while minimizing the inter-cell interference,which is represented by the second term Σ_(j)√{square root over(ρ_(f))}H_(j,k)Σ_(m≠k) ^(N*K)w_(j,m)a_(j,m) in the equation (Eq. 5), forthe client devices located in the overlapping coverage areas222(1)-222(4). However, given that the large-scale factor of H_(j,k) maynot be similar at the same time outside the overlapping coverage areas222(1)-222(4), the combination gain for the desired wirelesscommunications signal may be small and the inter-cell interferences forthe client devices located outside the overlapping coverage areas222(1)-222(4) may become significant. In this regard, it may be desiredto enhance the conventional network MIMO system 214 to mitigateinter-cell interferences for the cell-boundary client devices, whilereducing computational overhead and complexity associated withprocessing the channel-data information related to all of the clientdevices 218(1)-218(17).

In this regard, FIG. 3 is a schematic diagram of an exemplary WDS 300 inwhich a plurality of remote units 302(1)-302(N) is configured to supportcooperative transmission in a plurality of coverage cells 304(1)-304(N)with reduced computational overhead and enhanced inter-cell interferencemitigation compared to the conventional network MIMO system 214 of FIG.2B. The coverage cells 304(1)-304(N) are determined by a plurality ofcell boundaries 305(1)-305(N), respectively. The remote units302(1)-302(N) include a plurality of antenna arrays 306(1)-306(N),respectively. Each of the antenna arrays 306(1)-306(N) includes aplurality of antennas 308(1)-308(M). In this regard, the remote units302(1)-302(N) can be configured to provide similar functionalities asthe RRHs 216(1)-216(4) in the conventional network MIMO system 214 ofFIG. 2B.

Although the remote units 302(1)-302(N) in the WDS 300 are functionallyequivalent to the RRHs 216(1)-216(4) in the conventional network MIMOsystem 214, the WDS 300 is distinctively different from the conventionalnetwork MIMO system 214 in several aspects. In one aspect, unlike theconventional network MIMO system 214, in which each of the clientdevices 218(1)-218(17) only communicates with one of the RRHs216(1)-214(4), multiple cooperative remote units among the remote units302(1)-302(N) can coordinate with each other to help mitigate inter-cellinterference for a client device(s) located in a cooperative area(s) byforming at least two RF beams to communicate a downlink signal(s) (e.g.,simultaneously) to the client device(s). In this regard, the clientdevice(s) would be communicating with at least two of the remote units302(1)-302(N). As a result, it is possible to improve the SINR of thedownlink signal(s) received by the client device(s), thus mitigatinginter-cell interference for the client device(s). In another aspect, asopposed to the conventional network MIMO system 214 in which the RRHs216(1)-216(4) cooperate with each other based on the channel-datainformation received from all of the client devices 218(1)-218(17), themultiple cooperative remote units in the WDS 300 cooperate solely basedon the channel-data information provided by the client device(s) in thecooperative area(s). Accordingly, it is also possible to reduceprocessing overhead and complexity for the multiple cooperative remoteunits, thus helping to improve robustness and performance of the WDS300.

For the convenience of reference and illustration, the remote units302(1)-302(4) and the coverage cells 304(1)-304(4) are discussed hereinas non-limiting examples. It should be appreciated that theconfiguration and operation principles discussed with reference to theremote units 302(1)-302(4) are generally applicable to all of the remoteunits 302(1)-302(N) in the WDS 300.

In a non-limiting example, the coverage cells 304(1)-304(4) areneighboring coverage cells that overlap, at least partially, with eachother in a plurality of overlapping coverage areas 310(1)-310(4). Eachof the remote units 302(1)-302(4) defines a home coverage cell among thecoverage cells 304(1)-304(4) to communicate one or more downlink signals312(1)-312(K) to one or more client devices 314 located within aboundary 316 of the home coverage cell. For example, the remote unit302(1) defines the coverage cell 304(1) as the home coverage cell andcommunicates the downlink signals 312(1)-312(K) to the client devices314 located within the boundary 316 of the coverage cell 304(1). In anon-limiting example, the coverage cells 304(2)-304(4) are neighboringcoverage cells to the home coverage cell 304(1). Accordingly, the remoteunits 302(2)-302(4) are cooperative remote units to the remote unit302(1), and the overlapping coverage areas 310(1)-310(4) are cooperativeareas for the remote units 302(1)-302(4). In this regard, the remoteunits 302(1)-302(4) are configured to coordinate with each other to helpmitigate inter-cell interferences for the client devices 314 located inthe overlapping coverage areas 310(1)-310(4).

In a non-limiting example, a selected remote unit 318, for example theremote unit 302(1), is configured to coordinate with a neighboringremote unit, for example the remote unit 302(2), to mitigate theinter-cell interference for at least one selected client device 320located in the overlapping coverage area 310(1). In this regard, thecoverage cell 304(1) is also the home coverage cell of the selectedremote unit 318, and the coverage cell 304(2) is the neighboringcoverage cell defined by the remote unit 302(2). Thus, the remote unit302(2) is the cooperative remote unit of the selected remote unit 318,and the overlapping coverage area 310(1) is the cooperative area for theselected remote unit 318. Notably, the selected client device 320 canalso be in any of the overlapping coverage areas 310(2)-310(4).Accordingly, any of the remote units 302(2)-302(4) can be theneighboring remote unit that cooperates with the selected remote unit318. Likewise, the selected remote unit 318 can also be any of theremote units 302(2)-302(4).

Continuing with the non-limiting example above, the selected remote unit318 includes a control circuit 321, which can be a microcontroller, amicroprocessor, or a field-programmable gate array (FPGA), for example.Notably, the control circuit 321 can be included in any of the remoteunits 302(1)-302(4). The control circuit 321 identifies the selectedclient device 320 located in the overlapping coverage area 310(1) andreceives channel-data information from the selected client device 320.For example, the selected remote unit 318 can receive the channel-datainformation based on at least one CSI provided by the selected clientdevice 320. The control circuit 321 in the selected remote unit 318 isconfigured to form a first RF beam 322 based on the channel-datainformation received from the selected client device 320 to distributeat least one first downlink signal 324 among the downlink signals312(1)-312(K) to the selected client device 320. The control circuit 321in the selected remote unit 318 is also configured to coordinate withthe neighboring remote unit 302(2) based on the channel-data informationreceived from the selected client device 320 to form a second RF beam326 to communicate the first downlink signal 324 from the neighboringremote unit 302(2) to the selected client device 320.

The control circuit 321 in the selected remote unit 318 coordinates witha respective control circuit in the neighboring remote unit 302(2) toform the first RF beam 322 and the second RF beam 326 based on a definedtime-frequency resource. In a non-limiting example, the definedtime-frequency resource includes an identical number of RF carriers(e.g., subcarriers) and an identical number of symbol streams. Theselected remote unit 318 and the neighboring remote unit 302(2) are eachconfigured to pre-code the first downlink signal 324 based on thechannel-data information to provide phase coherency between the first RFbeam 322 and the second RF beam 326. In a non-limiting example, thephase coherency between the first RF beam 322 and the second RF beam 326means that the first RF beam 322 and the second RF beam 326 maintain aconstant phase difference, thus allowing the first RF beam 322 and thesecond RF beam 326 to be linearly combined at the selected client device320. As such, the selected client device 320 can receive the first RFbeam 322 and the second RF beam 326 (e.g., simultaneously) from theselected remote unit 318 and the neighboring remote unit 302(2).Accordingly, the selected client device 320 may linearly combine thefirst downlink signal 324 received via the first RF beam 322 and thesecond RF beam 326 to improve the SINR of the first downlink signal 324.As a result, the inter-cell interference for the selected client device320 may be mitigated.

The selected remote unit 318 can be configured to cooperate with theneighboring remote unit 302(2) according to a process. In this regard,FIG. 4 is a flowchart of an exemplary process 400 that can be employedby the selected remote unit 318 in FIG. 3 to support cooperativetransmission in the WDS 300. According to the process 400, the selectedremote unit 318 identifies the selected client device 320 located in theoverlapping coverage area 310(1) between the home coverage cell 304(1)of the selected remote unit 318 and the neighboring coverage cell 304(2)defined by the neighboring remote unit 302(2) (block 402). The selectedremote unit 318 receives the channel-data information from the selectedclient device 320 (block 404). The selected remote unit 318 forms thefirst RF beam 322 based on the channel-data information to distributethe first downlink signal 324 to the selected client device 320 (block406). The selected remote unit 318 coordinates with the neighboringremote unit 302(2) based on the channel-data information to form thesecond RF beam 326 to distribute the first downlink signal 324 from theneighboring remote unit 302(2) to the selected client device 320 (block408).

With reference back to FIG. 3, the selected remote unit 318 is alsoconfigured to identify at least one second selected client device 328located within the boundary 316 of the home coverage cell 304(1) of theselected remote unit 318, but outside the overlapping coverage area310(1). The selected remote unit 318 is further configured to distributeat least one second downlink signal 330 among the downlink signals312(1)-312(K) to the second selected client device 328. The selectedremote unit 318 may distribute the second downlink signal 330 (e.g.,simultaneously) with the first downlink signal 324 based on the definedtime-frequency resource. The selected remote unit 318 may pre-code thesecond downlink signal 330 to provide spatial separation from the firstdownlink signal 324, thus helping to mitigate intra-cell interferencebetween the first downlink signal 324 and the second downlink signal330.

In the WDS 300, a transmitted signal s_(i) communicated by the remoteunit 302(i) (1≤i≤N) can be expressed in the equation (Eq. 6) below.

$\begin{matrix}{s_{i} = {{W_{i}a_{i}} = {{\sum_{k = 1}^{K + L}{w_{i,k}a_{i,k}}} = {\begin{bmatrix}w_{i,1} & w_{i,2} & \cdots & w_{i,{K + L}}\end{bmatrix}{\quad{\begin{bmatrix}a_{i,1} \\a_{i\; 2} \\\vdots \\a_{i,{K + L}}\end{bmatrix} = {\begin{bmatrix}w_{i,1,1} & w_{i,2,1} & \cdots & w_{i,{K + L},1} \\w_{i,1,2} & w_{i,2,2} & \cdots & w_{i,{K + L},2} \\\vdots & \vdots & \ddots & \vdots \\w_{i,1,M} & w_{i,2,M} & \cdots & w_{i,{K + L},M}\end{bmatrix}\begin{bmatrix}a_{i,1} \\a_{i\; 2} \\\vdots \\a_{i,{K + L}}\end{bmatrix}}}}}}}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

In the equation (Eq. 6) above, W_(i) represents an M×(K+L) pre-codingmatrix for remote unit 302(i), in which M represents the number of theantennas 308(1)-308(M) in the remote unit 302(i), K represents number ofthe client devices 314 in the coverage cell 304(i), and L representsnumber of the client devices 314 located in an overlapping coverage areadefined by a neighboring remote unit cooperating with the remote unit302(i). In addition, a_(i), represents a (K+L)×1 transmitted symbolvector for the RRH 216(i), w_(i,k) represents an M×1 pre-coding vectorof a client device 314(k) (1≤k≤K), and a_(i,k) represents transmittedsymbol for the client device 314(k).

A received signal x_(i,k) received by an in-cell client device 314(k),such as the second selected client device 328 located in the homecoverage cell 304(1) and outside the overlapping coverage area 310(1),can be expressed in the equation (Eq. 7) below.

x _(i,k)=√{square root over (ρ_(f))}H _(i,k) w _(i,k) a _(i,k)+√{squareroot over (ρ_(f))}H _(i,k)Σ_(m≠k) ^(K+L) w _(i,m) a _(i,m)+√{square rootover (ρ_(f))}Σ_(j≠i) ⁴Σ_(m=1) ^(K+L) H _(j,k) w _(j,m) a _(j,m) +n_(i)  (Eq. 7)

A received signal x_(i,k) received by a cell-boundary client device314(k), such as the selected client device 320 located in theoverlapping coverage area 310(1), can be expressed in the equation (Eq.8) below.

x _(i,k)=Σ_(j) ^(Ncop)√{square root over (ρ_(f))}H _(j,k) w _(j,k) a_(j,k)+Σ_(j)√{square root over (ρ_(f))}H _(j,k)Σ_(m≠K+k) ^(K+L) w _(j,m)a _(j,m) +n _(i)  (Eq. 8)

In the equation (Eq. 8) above, N_(cop) represents the number ofcooperative remote units for a client device 314(k). For example, forthe selected client device 320, the selected remote unit 318 and theneighboring remote unit 302(2) are cooperating in the overlappingcoverage area 310(1). In this regard, N_(cop) equals two (2) for theselected client device 320. Given that the selected remote unit 318coordinates with the neighboring remote unit 302(2) based on thechannel-data information received from the selected client device 320,the large-scale factor of H_(j,k) for the selected remote unit 318 andthe neighboring remote unit 302(2) would be similar. Thus, assuming thatthe selected client device 320 is indexed as K+k among the clientdevices 314 in the coverage cell 304(1), the combined gain of the firstdownlink signal 324, which is represented by the term Σ_(j) ^(Ncop)√{square root over (ρ_(f))}H_(j,k)w_(j,k)a_(j,k) in the equation (Eq.8), can be maximized. In addition, the inter-cell interferencerepresented by the term Σ_(j)√{square root over (ρ_(f))}H_(j,k)Σ_(m≠K+k)^(K+L)w_(j,m)a_(j,m) can be minimized.

Performance of the WDS 300 may be validated based on simulations. Inthis regard, FIG. 5A is a graph 500 providing an exemplary illustrationof user throughput comparison between the conventional network MIMOsystem 214 of FIG. 2B and the WDS 300 of FIG. 3 in which the remoteunits 302(1)-302(N) cooperate based on MF pre-coding.

The graph 500 includes a first curve 502 and a second curve 504. Thefirst curve 502 represents user throughput in the conventional networkMIMO system 214, and the second curve 504 represents user throughput inthe WDS 300. The first curve 502 and the second curve 504 are plottedbased on a system level simulation configured according to parameterslisted in Table 1 below.

TABLE 1 Parameters Assumption Coverage Area 50-meter × 50-meter, with 4remote units Antenna 50 antennas per remote unit Carrier FrequencyCenter Frequency (CF) = 2 GHz Channel Model Pathloss model: WinnerIndoor Office Small Scale Fading: independent and identicallydistributed (i.i.d.) Number of Client Devices 40 Client Device Speed ofInterest 3 kilometers per hour (km/h) Total Transmit Power (P_(total))17 dBm, 20 MHz carrier Thermal Density Power −174 dBm Noise Figure   9dB Scheduling Algorithm Full bandwidth scheduling Channel EstimationPrecise

The system level simulation is conducted by randomly dropping the clientdevices into the 50-meter×50-meter coverage area and based on long-termevolution (LTE) with massive MIMO incorporated into the simulationsystem.

FIG. 5B is a graph 506 providing an exemplary illustration of userthroughput comparison between the conventional network MIMO system 214of FIG. 2B and the WDS 300 of FIG. 3 in which the remote units302(1)-302(N) cooperate based on zero-forcing (ZF) pre-coding.

The graph 506 includes a first curve 508 and a second curve 510. Thefirst curve 508 represents user throughput in the conventional networkMIMO system 214, and the second curve 510 represents user throughput inthe WDS 300. The first curve 508 and the second curve 510 are plottedbased on the system level simulation configured according to parameterslisted in the Table 1 above.

Simulation result and comparisons as plotted in FIGS. 5A-5B aresummarized in Table 2 below.

TABLE 2 Remote Unit 5% User Average Transmission Throughput ThroughputPrecoding Method (Mbps) (Mbps) MF Conventional 1.8 31.38 Network MIMOSystem (214) WDS (300) 2.45 32.74 Performance Gain 36.11% 4.34% ZFConventional 3 80.86 Network MIMO System (214) WDS (300) 4.5 89.36Performance Gain   50% 10.5%

FIG. 6 is a schematic diagram of an exemplary WDS 600 provided in theform of an optical fiber-based WDS that can include the remote units302(1)-302(N) of FIG. 3 for supporting cooperative transmission in theWDS 300. The WDS 600 includes an optical fiber for distributingcommunications services for multiple frequency bands. The WDS 600 inthis example is comprised of three (3) main components. A plurality ofradio interfaces provided in the form of radio interface modules (RIMs)602(1)-602(M) are provided in a central unit 604 to receive and processa plurality of downlink communications signals 606D(1)-606D(R) prior tooptical conversion into downlink optical fiber-based communicationssignals. The downlink communications signals 606D(1)-606D(R) may bereceived from a base station as an example. Each of the downlinkcommunications signals 606D(1)-606D(R) may include the downlink signals312(1)-312(K) in FIG. 3 to be distributed by each of the remote units302(1)-302(N). The RIMs 602(1)-602(M) provide both downlink and uplinkinterfaces for signal processing. The notations “1-R” and “1-M” indicatethat any number of the referenced component, 1-R and 1-M, respectively,may be provided. The central unit 604 is configured to accept the RIMs602(1)-602(M) as modular components that can easily be installed andremoved or replaced in the central unit 604. In one example, the centralunit 604 is configured to support up to twelve (12) RIMs 602(1)-602(12).Each of the RIMs 602(1)-602(M) can be designed to support a particulartype of radio source or range of radio sources (i.e., frequencies) toprovide flexibility in configuring the central unit 604 and the WDS 600to support the desired radio sources.

For example, one RIM 602 may be configured to support the PersonalizedCommunications System (PCS) radio band. Another RIM 602 may beconfigured to support the 800 megahertz (MHz) radio band. In thisexample, by inclusion of the RIMs 602(1)-602(M), the central unit 604could be configured to support and distribute communications signals onboth PCS and Long-Term Evolution (LTE) 700 radio bands, as an example.The RIMs 602(1)-602(M) may be provided in the central unit 604 thatsupport any frequency bands desired, including, but not limited to, theUS Cellular band, PCS band, Advanced Wireless Service (AWS) band, 700MHz band, Global System for Mobile communications (GSM) 900, GSM 1800,and Universal Mobile Telecommunications System (UMTS). The RIMs602(1)-602(M) may also be provided in the central unit 604 that supportany wireless technologies desired, including, but not limited to, CodeDivision Multiple Access (CDMA), CDMA200, 1×RTT, Evolution—Data Only(EV-DO), UMTS, High-speed Packet Access (HSPA), GSM, General PacketRadio Services (GPRS), Enhanced Data GSM Environment (EDGE), TimeDivision Multiple Access (TDMA), LTE, iDEN, and Cellular Digital PacketData (CDPD).

The RIMs 602(1)-602(M) may be provided in the central unit 604 thatsupport any frequencies desired, including, but not limited to, US FCCand Industry Canada frequencies (824-849 MHz on uplink and 869-894 MHzon downlink), US FCC and Industry Canada frequencies (1850-1915 MHz onuplink and 1930-1995 MHz on downlink), US FCC and Industry Canadafrequencies (1710-1755 MHz on uplink and 2110-2155 MHz on downlink), USFCC frequencies (698-716 MHz and 776-787 MHz on uplink and 728-746 MHzon downlink), EU R & TTE frequencies (880-915 MHz on uplink and 925-960MHz on downlink), EU R & TTE frequencies (1710-1785 MHz on uplink and1805-1880 MHz on downlink), EU R & TTE frequencies (1920-1980 MHz onuplink and 2110-2170 MHz on downlink), US FCC frequencies (806-824 MHzon uplink and 851-869 MHz on downlink), US FCC frequencies (896-901 MHzon uplink and 929-941 MHz on downlink), US FCC frequencies (793-805 MHzon uplink and 763-775 MHz on downlink), and US FCC frequencies(2495-2690 MHz on uplink and downlink).

With continuing reference to FIG. 6, the downlink communications signals606D(1)-606D(R) are provided to a plurality of optical interfacesprovided in the form of optical interface modules (OIMs) 608(1)-608(N)in this embodiment to convert the downlink communications signals606D(1)-606D(R) into a plurality of downlink optical fiber-basedcommunications signals 610D(1)-610D(R). The notation “1-N” indicatesthat any number of the referenced component 1-N may be provided. TheOIMs 608(1)-608(N) may be configured to provide a plurality of opticalinterface components (OICs) that contain optical-to-electrical (O/E) andelectrical-to-optical (E/O) converters, as will be described in moredetail below. The OIMs 608(1)-608(N) support the radio bands that can beprovided by the RIMs 602(1)-602(M), including the examples previouslydescribed above.

The OIMs 608(1)-608(N) each include E/O converters to convert thedownlink communications signals 606D(1)-606D(R) into the downlinkoptical fiber-based communications signals 610D(1)-610D(R). The downlinkoptical fiber-based communications signals 610D(1)-610D(R) arecommunicated over a downlink optical fiber-based communications medium612D to a plurality of remote units 614(1)-614(S). In a non-limitingexample, the remote units 614(1)-614(S) can be provided as or replacedby the remote units 302(1)-302(N) for supporting cooperativetransmission. The notation “1-S” indicates that any number of thereferenced component 1-S may be provided. Remote unit O/E convertersprovided in the remote units 614(1)-614(S) convert the downlink opticalfiber-based communications signals 610D(1)-610D(R) back into thedownlink communications signals 606D(1)-606D(R) which are then convertedinto a plurality of downlink RF communications signals and provided toantennas 616(1)-616(S) in the remote units 614(1)-614(S) to clientdevices in the reception range of the antennas 616(1)-616(S).

The remote units 614(1)-614(S) receive a plurality of uplink RFcommunications signals from the client devices through the antennas616(1)-616(S). The remote units 614(1)-614(S) convert the uplink RFcommunications signals into a plurality of uplink communications signals618U(1)-618U(S). Remote unit E/O converters are also provided in theremote units 614(1)-614(S) to convert the uplink communications signals618U(1)-618U(S) into a plurality of uplink optical fiber-basedcommunications signals 610U(1)-610U(S). The remote units 614(1)-614(S)communicate the uplink optical fiber-based communications signals610U(1)-610U(S) over an uplink optical fiber-based communications medium612U to the OIMs 608(1)-608(N) in the central unit 604. The OIMs608(1)-608(N) include O/E converters that convert the received uplinkoptical fiber-based communications signals 610U(1)-610U(S) into aplurality of uplink communications signals 620U(1)-620U(S) which areprocessed by the RIMs 602(1)-602(M) and provided as the uplinkcommunications signals 620U(1)-620U(S). The central unit 604 may providethe uplink communications signals 620U(1)-620U(S) to a base station orother communications system.

Note that the downlink optical fiber-based communications medium 612Dand the uplink optical fiber-based communications medium 612U connectedto each of the remote units 614(1)-614(S) may be a common opticalfiber-based communications medium, wherein for example, wave divisionmultiplexing (WDM) is employed to provide the downlink opticalfiber-based communications signals 610D(1)-610D(R) and the uplinkoptical fiber-based communications signals 610U(1)-610U(S) on the sameoptical fiber-based communications medium.

The WDS 600 of FIG. 6 may be provided in an indoor environment, asillustrated in FIG. 7. FIG. 7 is a partial schematic cut-away diagram ofan exemplary building infrastructure 700 in which a WDS, such as the WDS600 of FIG. 6, includes the remote units 302(1)-302(N) of FIG. 3 forsupporting cooperative transmission in the WDS 300. The buildinginfrastructure 700 in this embodiment includes a first (ground) floor702(1), a second floor 702(2), and a third floor 702(3). The floors702(1)-702(3) are serviced by a central unit 704 to provide antennacoverage cells 706 in the building infrastructure 700. The central unit704 is communicatively coupled to a base station 708 to receive downlinkcommunications signals 710D from the base station 708. The central unit704 is communicatively coupled to a plurality of remote units 712 todistribute the downlink communications signals 710D to the remote units712 and to receive uplink communications signals 710U from the remoteunits 712, as previously discussed above. The downlink communicationssignals 710D and the uplink communications signals 710U communicatedbetween the central unit 704 and the remote units 712 are carried over ariser cable 714. The riser cable 714 may be routed through interconnectunits (ICUs) 716(1)-716(3) dedicated to each of the floors 702(1)-702(3)that route the downlink communications signals 710D and the uplinkcommunications signals 710U to the remote units 712 and also providepower to the remote units 712 via array cables 718.

FIG. 8 is a schematic diagram representation of additional detailillustrating an exemplary computer system 800 that could be employed ina controller, such as the control circuit 321 employed by any of theremote units 302(1)-302(N) of FIG. 3, for supporting cooperativetransmission in the WDS 300. In this regard, the computer system 800 isadapted to execute instructions from an exemplary computer-readablemedium to perform these and/or any of the functions or processingdescribed herein.

In this regard, the computer system 800 in FIG. 8 may include a set ofinstructions that may be executed to predict frequency interference toavoid or reduce interference in a multi-frequency distributed antennasystem (DAS). The computer system 800 may be connected (e.g., networked)to other machines in a local area network (LAN), an intranet, anextranet, or the Internet. While only a single device is illustrated,the term “device” shall also be taken to include any collection ofdevices that individually or jointly execute a set (or multiple sets) ofinstructions to perform any one or more of the methodologies discussedherein. The computer system 800 may be a circuit or circuits included inan electronic board card, such as a printed circuit board (PCB), aserver, a personal computer, a desktop computer, a laptop computer, apersonal digital assistant (PDA), a computing pad, a mobile device, orany other device, and may represent, for example, a server or a user'scomputer.

The exemplary computer system 800 in this embodiment includes aprocessing circuit or processor 802, a main memory 804 (e.g., read-onlymemory (ROM), flash memory, dynamic random access memory (DRAM), such assynchronous DRAM (SDRAM), etc.), and a static memory 806 (e.g., flashmemory, static random access memory (SRAM), etc.), which may communicatewith each other via a data bus 808. Alternatively, the processor 802 maybe connected to the main memory 804 and/or the static memory 806directly or via some other connectivity means. The processor 802 may bea controller, and the main memory 804 or the static memory 806 may beany type of memory.

The processor 802 represents one or more general-purpose processingdevices, such as a microprocessor, central processing unit, or the like.More particularly, the processor 802 may be a complex instruction setcomputing (CISC) microprocessor, a reduced instruction set computing(RISC) microprocessor, a very long instruction word (VLIW)microprocessor, a processor implementing other instruction sets, orother processors implementing a combination of instruction sets. Theprocessor 802 is configured to execute processing logic in instructionsfor performing the operations and steps discussed herein.

The computer system 800 may further include a network interface device810. The computer system 800 also may or may not include an input 812,configured to receive input and selections to be communicated to thecomputer system 800 when executing instructions. The computer system 800also may or may not include an output 814, including, but not limitedto, a display, a video display unit (e.g., a liquid crystal display(LCD) or a cathode ray tube (CRT)), an alphanumeric input device (e.g.,a keyboard), and/or a cursor control device (e.g., a mouse).

The computer system 800 may or may not include a data storage devicethat includes instructions 816 stored in a computer-readable medium 818.The instructions 816 may also reside, completely or at least partially,within the main memory 804 and/or within the processor 802 duringexecution thereof by the computer system 800, the main memory 804 andthe processor 802 also constituting a computer-readable medium. Theinstructions 816 may further be transmitted or received over a network820 via the network interface device 810.

While the computer-readable medium 818 is shown in an exemplaryembodiment to be a single medium, the term “computer-readable medium”should be taken to include a single medium or multiple media (e.g., acentralized or distributed database, and/or associated caches andservers) that store the one or more sets of instructions. The term“computer-readable medium” shall also be taken to include any mediumthat is capable of storing, encoding, or carrying a set of instructionsfor execution by the processing device and that cause the processingdevice to perform any one or more of the methodologies of theembodiments disclosed herein. The term “computer-readable medium” shallaccordingly be taken to include, but not be limited to, solid-statememories, optical medium, and magnetic medium.

The embodiments disclosed herein include various steps. The steps of theembodiments disclosed herein may be formed by hardware components or maybe embodied in machine-executable instructions, which may be used tocause a general-purpose or special-purpose processor programmed with theinstructions to perform the steps. Alternatively, the steps may beperformed by a combination of hardware and software.

The embodiments disclosed herein may be provided as a computer programproduct, or software, that may include a machine-readable medium (orcomputer-readable medium) having stored thereon instructions, which maybe used to program a computer system (or other electronic devices) toperform a process according to the embodiments disclosed herein. Amachine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes: amachine-readable storage medium (e.g., ROM, random access memory(“RAM”), a magnetic disk storage medium, an optical storage medium,flash memory devices, etc.); and the like.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps, or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is in no way intendedthat any particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications, combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A communications system, comprising: a pluralityof remote units comprising optical-to-electrical converters andelectrical-to-optical converters, each configured to define a homecoverage cell among a plurality of coverage cells in the communicationssystem to communicate one or more downlink signals to one or more clientdevices located within a boundary of the home coverage cell; and acentral unit comprising electrical-to-optical converters andoptical-to-electrical converters and communicatively coupled to theplurality of remote units over an optical fiber-based communicationsmedium, wherein a selected remote unit among the plurality of remoteunits is configured to: identify at least one selected client devicelocated in at least one overlapping coverage area between the homecoverage cell of the selected remote unit and at least one neighboringcoverage cell defined by at least one neighboring remote unit among theplurality of remote units; receive channel-data information from the atleast one selected client device; form a first radio frequency (RF) beambased on the channel-data information to distribute at least one firstdownlink signal among the one or more downlink signals to the at leastone selected client device; and coordinate with the at least oneneighboring remote unit based on the channel-data information to form atleast one second RF beam to distribute the at least one first downlinksignal from the at least one neighboring remote unit to the at least oneselected client device, and the selected remote unit and the at leastone neighboring remote unit are configured to form the respective firstRF beam and the at least one second RF beam based on a definedtime-frequency resource.
 2. The communications system of claim 1,wherein the selected remote unit is further configured to: identify atleast one second selected client device located within the boundary ofthe home coverage cell of the selected remote unit and outside the atleast one overlapping coverage area; and distribute at least one seconddownlink signal among the one or more downlink signals to the at leastone second selected client device.
 3. The communications system of claim2, wherein the selected remote unit and the at least one neighboringremote unit are each configured to pre-code the first downlink signalbased on the channel-data information to provide phase coherency betweenthe at least one first RF beam and the at least one second RF beam. 4.The communications system of claim 2, wherein the selected remote unitis further configured to receive the channel-data information based onat least one channel state information (CSI) provided by the at leastone selected client device.
 5. The communications system of claim 2,wherein the selected remote unit is further configured to distribute theat least one first downlink signal and the at least one second downlinksignal.
 6. The communications system of claim 2, wherein the selectedremote unit is further configured to distribute the at least one seconddownlink signal based on the defined time-frequency resource.
 7. Thecommunications system of claim 6, wherein the defined time-frequencyresource comprises an identical number of RF carriers and an identicalnumber of symbol streams.
 8. The communications system of claim 2,wherein the selected remote unit is further configured to pre-code theat least one second downlink signal to provide spatial separation fromthe at least one first downlink signal.
 9. A method for supportingcooperative transmissions in a communications system comprising acentral unit and a plurality of remote units distributed over multiplefloors of a building infrastructure, the method comprising: identifyingat least one selected client device located in at least one overlappingcoverage area between a home coverage cell of a selected remote unitamong the plurality of remote units and at least one neighboringcoverage cell defined by at least one neighboring remote unit among theplurality of remote units; receiving channel-data information from theat least one selected client device; forming a first radio frequency(RF) beam based on the channel-data information to distribute at leastone first downlink signal to the at least one selected client device;coordinating with the at least one neighboring remote unit based on thechannel-data information to form at least one second RF beam todistribute the at least one first downlink signal from the at least oneneighboring remote unit to the at least one selected client device;identifying at least one second selected client device located within aboundary of the home coverage cell of the selected remote unit andoutside the at least one overlapping coverage area; and distributing atleast one second downlink signal to the at least one second selectedclient device.
 10. The method of claim 9, further comprising forming therespective first RF beam and the at least one second RF beam based on adefined time-frequency resource.
 11. The method of claim 10, furthercomprising pre-coding the at least one first downlink signal based onthe channel-data information to provide phase coherency between thefirst RF beam and the at least one second RF beam.
 12. The method ofclaim 10, further comprising receiving the channel-data informationbased on at least one channel state information (CSI) provided by the atleast one selected client device.
 13. The method of claim 11, furthercomprising distributing the at least one first downlink signal and theat least one second downlink signal.
 14. The method of claim 13, whereinthe communications system comprises a downlink optical fiber-basedcommunications medium and an uplink optical fiber-based communicationsmedium.
 15. The method of claim 14, wherein the central unit compriseselectrical-to-optical and a plurality of optical-to-electricalconverters.